Asymmetric cell divisions, in which a cell divides to give two daughters with different fates, play an important role in the development of all multicellular organisms. In plants, because there is no cell migration, the regulation of asymmetric cell divisions is of heightened importance in determining organ morphology. In contrast to animal embryogenesis, most plant organs are not formed during embryogenesis. Rather, cells that form the apical meristems are set aside at the shoot and root poles. These reservoirs of stem cells are considered to be the source of all post-embryonic organ development in plants. A fundamental question in developmental biology is how meristems function to generate plant organs.
2.1. Root Development
Root organization is established during embryogenesis. This organization is propagated during postembryonic development by the root meristem. Following germination, the development of the postembryonic root is a continuous process, wherein a series of initials or stem cells continuously divide to perpetuate the pattern established in the embryonic root (Steeves & Sussex, 1972, Patterns in Plant Development, Englewood Cliffs, N.J.: Prentice-Hall, Inc.).
2.1.1. Arabidopsis Root Development
Due to the organization of the Arabidopsis root, it is possible to follow the fate of cells from the meristem to maturity and identify the progenitors of each cell type (Dolan et al., 1993, Development 119:71-84). The Arabidopsis root is a relatively simple and well characterized organ. The radial organization of the mature tissues in the Arabidopsis root has been likened to tree rings with the epidermis, cortex, endodermis and pericycle forming radially symmetric cell layers that surround the vascular cylinder. See also Dolan et al, 1993, Development 119:71-84. These mature tissues are derived from four sets of stem cells or initials: i) the columella root cap initial; ii) the pericycle/vascular initial; iii) the epidermal/lateral root cap initial; and iv) the cortex/endodermal initial (Dolan et al., 1993, Development 119:71-84). It has been shown that these initials undergo asymmetric divisions (Scheres et al., 1995, Development 121:53-62). The cortex/endodermal initial, for example, first divides anticlinally (in a transverse orientation). This asymmetric division produces another initial and a daughter cell. The daughter cell, in turn, expands and then divides periclinally (in the longitudinal orientation). This second asymmetric division produces the progenitors of the endodermis and the cortex cell lineages.
Furthermore, root radial organization in Arabidopsis is produced by three distinct developmental strategies. First, primary roots employ stem cells, wherein initials undergo asymmetric divisions first to regenerate themselves and then to generate the cell lineages of the root. Second, in the embryo, sequential asymmetric divisions subdivide pre-existing tissue to form the cell layers of the embryonic root. Finally, lateral roots are formed by a strategy of cell proliferation that originates in differentiated tissues. Remarkably, within a given species, all three strategies result in roots with a nearly identical radial organization.
2.2. Genes Regulating Root Structure
Mutations that disrupt the asymmetric divisions of the cortex/endodermal initial have been identified and characterized (Benfey et al., 1993, Development 119:57-70; Scheres et al., 1995, Development 121:53-62). short-root (shr) and scarecrow (scr) mutants are missing a cell layer between the epidermis and the pericycle. In both types of mutants, the cortex/endodermal initial divides anticlinally, but the subsequent periclinal division that increases the number of cell layers does not take place (Benfey et al., 1993, Development 119:57-70; Scheres et al., 1995, Development 121:53-62). The defect is first apparent in the embryo and it extends throughout the entire embryonic axis, which includes the embryonic root and hypocotyl (Scheres et al., 1995, Development 121:53-62). This is true also for other radial organization mutants characterized to date, suggesting that radial patterning that occurs during embryonic development may influence the post-embryonic pattern generated by the meristematic initials (Scheres et al., 1995, Development 121:53-62).
In embryos, cortex and endodermis are also formed from the asymmetric division of embryonic ground tissue at the early torpedo stage. This division occurs along the length of the embryonic axis which encompasses the embryonic root and hypocotyl. In both scarecrow and short-root, the embryonic ground tissue fails to undergo the asymmetric division into cortex and endodermis. Hence, these two mutations identify genes required for the asymmetric division that produces cortex and endodermis from ground tissue in the embryo and from the cortex/endodermal initials in primary and lateral roots.
Characterization of the mutant cell layer in shr indicated that two endodermal-specific markers were absent, while the cortex-specific markers were present, indicating that the mutant layer has differentiated attributes only of cortex. Thus, in short-root the initial cell divides transversely, then fails to make the longitudinal division and in the resulting cell only the cortex differentiation program is activated. This suggests that the short-root mutant phenotype is equivalent to the loss of the endodermal cell layer and distinct from the scarecrow phenotype. For SHORT-ROOT, the findings suggest that it is involved in specification of endodermis identity and is also directly or indirectly required for the asymmetric division to form cortex and endodermis. (Benfey et al., 1993, Development 119:57-70).
2.3. Role of SHR in Radial Patterning of the Shoot
SHR functions are not confined to the roots. The hypocotyls of SHR mutants are also missing one of the ground tissue derived cell layers (Scheres, B., et al., 1995, Development 121, 53-62; Fukaki et al., 1998, Plant J. 14, 425-430) but the cellular identities of the remaining cell layers are not known. The absence of normal hypocotyl and shoot endodermis correlates with an agravitropic phenotype in both hypocotyl and shoot inflorescence (Fukaki, et al 1998, Plant J. 14, 425-430). Thus, mutations in the SHR genes lead to a radial pattern deficiency both in the roots and shoots. However, the pattern defects have no effect on root gravitropism. The data suggests that lack of sedimenting amyloplasts in the shr mutant stems is responsible for the shoot agravitropic phenotype (Fukaki, et al, 1998, Plant J. 14, 425-430).
2.4. Geotropism
In plants, the capacity for gravitropism has been correlated with the presence of amyloplast sedimentation. See, e.g., Volkmann and Sievers, 1979, Encyclopedia Plant Physiol., N.S. vol 7, pp. 573-600; Sack, 1991, Intern. Rev. Cytol. 127:193-252; Björkmann, 1992, Adv. Space Res. 12:195-201; Poff et al., in The Physiology of Tropisms, Meyerowitz & Somerville (eds); Cold Spring Harbor Laboratory Press, Plainview, N.Y. (1994) pp. 639-664; Barlow, 1995, Plant Cell Environ. 18:951-962. Amyloplast sedimentation only occurs in cells in specific locations at distinct developmental stages. That is, when and where sedimentation occurs is precisely regulated (Sack, 1991, Intern. Rev. Cytol. 127:193-252). In roots, amyloplast sedimentation only occurs in the central (columella) cells of the rootcap; as these cells mature into peripheral cap cells, the amyloplasts no longer sediment (Sack & Kiss, 1989, Amer. J. Bot. 76:454464; Sievers & Braun, in The Root Cap: Structure and Function, Wassail et al. (eds.), New York: M. Dekker (1996) pp. 31-49). In stems of many plants, including Arabidopsis, amyloplast sedimentation occurs in the starch sheath (endodermis) especially in elongating regions of the stem (von Guttenberg, Die Physiologischen Scheiden, Handbuch der Pflanzenanatomie; K. Linsbauer (ed.), Berlin: Gebruder Borntraeger, vol. 5 (1943) p. 217; Sack, 1987, Can. J. Bot. 65:1514-1519; Sack, 1991, Intern. Rev. Cytol. 127:193-252; Caspar & Pickard, 1989, Planta 177:185-197; Volkmann et al., 1993, J. Pl. Physiol. 142:710-6).
Gravitropic mutants have been studied for evidence that proves the role of amyloplast sedimentation in gravity sensing. However, many gravitropic mutations affect downstream events such as auxin sensitivity or metabolism (Masson, 1995, BioEssays 17:119-127). Other mutations seem to affect gene products that process information from gravity sensing. For example, the lazy mutants of higher plants and comparable mutants in mosses can clearly sense and respond to gravity, but the mutations reverse the normal polarity of the gravitropic response (Gaiser & Lomax, 1993, Plant Physiol. 102:339-344; Jenkins et al., 1986, Plant Cell Environ 9:637-644). Other mutations appear to affect gravitropism of specific organs. For example, sgr mutants have defective shoot gravitropism (Fukaki et al., 1996, Plant Physiol. 110:933-943; Fukaki et al., 1996, Plant Physiol. 110:945-955; Fukaki et al., 1996, Plant Res. 109:129-137).
Citation or identification of any reference herein shall not be construed as an admission that such reference is available as prior art to the present invention.